FIG. 1 shows a thermal head made by partly providing a glass glaze layer 2 on an alumina or other insulative substrate 1, and sequentially overlaid thereon with a heating resistor layer 4 and a power supply conductive layer 5 via an undercoat layer 3. The power supply conductive layer 5 is patterned with independent electrodes (only one is shown) 5a and a common electrode 5b. Between the independent electrodes 5a and the common electrode 5b is exposed the heating resistor layer 4 to form a heater arrangement A. The upper surface of the thermal head is protected by an overcoat layer 6.
The overcoat layer 6 not only prevents oxidization of the heating resistor 4 but also protects the heater arrangement A against various attacks.
It is known to use a silicon-aluminum-oxygen-nitrogen compound (hereinafter called "SIALON") as a material of the overcoat layer 6.
Since SIALON has a high Mohs hardness as much as 9 to 10 exhibiting an excellent strength against abrasion when used as a film, it is possible to use a SIALON layer alone to form the overcoat layer 6 to minimize the thickness thereof. Beside this, SIALON has an excellent heat transmission property. Therefore, such a thermal head including the SIALON overcoat layer 6 exhibits an excellent heat response. Moreover, since the SIALON film is strong against thermal attacks, it enables a great increase in the printing speed.
In the thermal head of this sort, however, since SIALON is poor in the adhesive power, adhesion between the heating resistor layer 4 and the overcoat layer 6 is not good in the heater arrangement A. Therefore, when the printer bites hard foreign matters during printing and any crack is produced in the overcoat layer 6, the layer 6 is stripped or separated away at the cracks so as to expose the heating resistor layer 4 against the exterior air. This greatly accelerates oxidization of the heating resistor layer 4 from the exposed portions thereof and increases the resistance. This invites a sudden decrease in heat so great to substantially unable printing. Because of these problems, the thermal head of this type cannot improve the printing life sufficiently and cannot make uniform the printing life.
Further, although the thermal head of this type has a good thermal response and enables a high speed printing, it consumes a large electric power during printing. Therefore, an increased running cost is required for a printer using the thermal head.